The “fog” you may sometimes experience on your glasses or goggles occurs when atmospheric humidity near the lens condenses; this happens due to a relatively significant discrepancy between the temperature of the lens and the surrounding air. As the surface attempts to reach an equilibrium between the two temperatures, heat energy is given up; and, as the energy within the gaseous water molecules decreases, they form into tiny water droplets on your lens, otherwise known as “fog.”

So how do we stop the fog from forming on your glasses? There are two primary types of substances used here- surfactants and hydrophilic ingredients.

Surfactants (or surface acting agents) are a broad group of compounds that include everything from saliva to baby shampoo to emulsifiers. (Incidentally, it is a surfactant that is the reason certain things taste disgusting after using toothpaste, see: Why Does Orange Juice Taste Awful After Brushing Your Teeth. Surfactants also play a key role in why dropping a mentos in something like Diet Coke produces such a strong geiser-like reaction.) These work by lowering the surface tension of water.

In the case of lens fog, which is essentially just a bunch of tiny water droplets, the surfactant’s ability to lower surface tension helps prevent tiny droplet formation. So instead of numerous tiny droplets forming all over your glasses, you get a very thin film of water that isn’t likely to interfere with your vision.

If you don’t have an anti-fog spray handy, in addition to human saliva and shampoo, other soaps, glycerin and even, as previously mentioned, certain substances in toothpaste, are surfactants and thus will effectively prevent fog from accumulating on a lens.

The other primary method for preventing fog is to cover the lens with a hydrophilic coating, which may include a polyvinyl alcohol, polymers, hydrogels and colloids. Meaning “water loving,” hydrophilic ingredients absorb water and thus ultimately spread it throughout the coating, similar to surfactants, preventing the numerous tiny droplets of visible “fog.”

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Bonus Facts:

• Until recently, neither method worked perfectly as surfactants had to be re-applied periodically, and hydrophilic coatings can eventually wear away (think of when a sponge gets saturated and will no longer absorb water) or never fully adhere to the lens, due to the lens, itself, having a hydrophobic (water hating) surface (such as anti-glare). In 2011, however, researchers with the Université Laval in Quebec announced that, by laying down different compounds in successive layers, that included a polyvinyl alcohol coating, they had supposedly created a permanent, hydrophilic, anti-fog coating. According to the patent grant (July 31, 2012), the process consisted of: “A first polymer layer resulting from covalently bonding a polyanhydride polymer to said surface; and a second polymer layer resulting from covalently bonding a polymer . . . of polyvinyl alcohol, partially hydrolyzed polyester, polyether and cellulose derivative . . . . A substrate having an anti-fog coating, as well as a process for preparing said anti-fog coating . . . .” Of course, they wouldn’t be the first to claim to have come up with a permanent solution to this problem, that turns out to just be a relatively long lasting one.
  
• More than just a convenience, effective anti-fog measures can go a long way to preventing workplace injuries. In fact, in the U.S. alone, 2,000 eye injuries serious enough to require medical attention occur each day. Sadly, most of these could have been prevented had the worker worn protective eyewear. According to a study published in Accident Analysis & Prevention, the single biggest reason workers reported that they did not wear such recommended protection was fogging. In addition, more than 25% of respondents who had a workplace injury reported that fogged protective eyewear contributed to their injury.